Histamine disappears rapidly from plasma because of its short half-life. Because a metabolite of histamine, N-methylhistamine, is stable and has a longer half-life, determination of its plasma concentration can be useful in retrospective determination of histamine release. In this study, we measured plasma histamine and N-methylhistamine concentrations after d-tubocurarine (dTc) administration to evaluate the use of plasma N-methylhistamine measurement for confirming histamine release. After the induction of anesthesia, five patients received dTc, 0.8 mg/kg intravenously. A radioimmunoassay kit was used to determine plasma histamine and N-methylhistamine before and 1, 3, 5, and 10 min after administration of dTc. Histamine released by the injection of dTc reached a maximum level at 1 min, but decreased rapidly, whereas N-methylhistamine increased at 1 min and remained increased for at least 10 min. Good correlations were found between histamine concentration at 1 min and N-methylhistamine concentrations at 1, 3, 5, and 10 min, especially r = 0.999 (n = 5) at 10 min. N-Methylhistamine measurement with this kit can ascertain histamine release retrospectively in a semiquantitative manner.